This invention relates to an optical disk used as a high density information memory medium including a video disk, a disk for digital codes and the like.
Optical disks mostly used nowadays are of an air-sandwich structure. This is a structure in which a thin film comprising a recording material is coated on a substrate of glass, plastics, etc., and, in the case of double sided media, two such substrates are bonded at the inner and the outer peripheral part respectively through the medium of a spacer, the recording films being made to face to face each other, to form an integrated body.
As the material of the above-mentioned spacer, there have been used up to now those which have a high rigidity of approximately the same magnitude as that of the substrate, which include, in the case of plastic substrates, for example, polymethyl methacrylate, polycarbonate and the like.
As the dynamic characteristics of an optical disk, there are restrictions as to the dynamic axial runout and the acceleration of dynamic axial runout of the disk, depending on the servo performance of an optical head. Although not yet standardized, there are set as a criteria, for example in an optical disk of 130 mm outside diameter, for example a value within the range of .+-.250 .mu.m (&lt;500 Hz) and .+-.0.2 .mu.m (&gt;1.5 KHz) for dynamic axial runout and 20 m/sec.sup.2 or less for acceleration of dynamic axial runout.
When the above-mentioned optical disk is allowed to run on an information recording-playback apparatus (hereinafter referred to as "drive"), the optical disk is usually rotated at a revolution speed of 200 to 3600 rpm by means of a spindle motor. In this case, the optical disk receives the vibration of the spindle motor, depending on the type and the quality of the drive, and the environment in which it is used; and there arises the possibility of a resonance phenomenon occurring when the resonance point of the optical disk coincides with the vibration frequency of the drive. Consequently, under certain circumstances, there is the apprehension that the dynamic axial runout and the acceleration of dynamic axial runout exceed specified values, going above the servo control range of the optical head, and resultantly the reading and writing of information become impossible.
Further, Japanese Patent Application (Kokai) No. 71146/84 discloses that an elastomeric material as a spacer of air-sandwich optical disk comprising a substrate and a reinforcing plate is used. This purpose is to prevent a distortion of the substrate caused by a difference of thermal expansion between the substrate and reinforcing plates.
However, the present inventors found that when the spacers formed of elastomeric material were used, the optical disk had not an excellent vibration restricting performance enough to satisfy in a practical use. Accordingly, an optical disk which is free from such a problem is eagerly desired.